U.S. Pat. No. 4,153,923 discloses continuity testing of at least one ground wire of a cable connected between a power center and a load. The power system couples a signal to multi-phase conductors through a multi-phase power filter, and at the load the signal is coupled to the ground wire through another multi-phase power filter. The signal returns through the ground wire where a current transformer couples the signal to a receiver. A decrease in amplitude or frequency beyond a preset threshold opens a circuit breaker and cuts off power to the load. Similarly, U.S. Pat. No. 4,075,675 transmits a signal over the ground conductor. A receiver receives the transmitted signal and when the signal is absent, a circuit breaker is opened. Signal transmission over two or more ground wires is subject to cross talk.
Like the '923 patent, U.S. Pat. No. 4,368,498 requires a closed loop and is limited to detection of interruption of the monitor signal. Of interest patents include ground fault detection system, and methods described in U.S. Pat. No. 4,529,929 which measure ground fault current and U.S. Pat. No. 6,327,124 where motor circuit insulation failure is detected prior to applying power.
What is needed to overcome the problems associated with prior art monitors is a tone monitoring system that does not require a closed loop system with a multi-channel grounding system, the system channel detecting the frequency of each load used in a multi-channel system to monitor plural ground, eliminating frequency crosstalk occurring in prior art systems. Additionally, the system channel detecting the frequency of each load used in a multi-channel system to monitor plural ground circuits notifying the other channels to disregard the connected load frequency.